Conventional internal combustion engines such as diesel engines and gasoline engines generate exhaust streams that include a number of different air pollutants. These air pollutants include gaseous components, such as carbon monoxide (CO) and nitrogen oxides (NOx), and solid particulate matter known as soot. Therefore, conventional engines are often implemented with exhaust treatment systems that include a plurality of different exhaust treatment modules. One such module, known as a selective catalytic reduction (SCR) module, combines a gaseous or liquid reductant (e.g., urea or ammonia) with the exhaust stream and then reacts the combined stream with a catalyst to convert NOx in the exhaust stream into diatomic nitrogen (N2) and water (H2O). The effectiveness of the SCR module can generally be improved by increasing the ratio of nitrogen dioxide (NO2) to nitrogen monoxide (NO). Therefore, another module, known as a diesel oxidation catalyst (DOC), is commonly implemented upstream of the SCR module to convert NO to NO2.
Yet another module, known as a particulate trap, is commonly implemented upstream of the SCR module to trap soot in the exhaust stream. One type of particulate trap utilized in conjunction with diesel engines is known as a diesel particulate filter (DPF). The soot accumulated within the DPF can be burned away through a process called regeneration.
A recent trend in exhaust treatment systems is to combine the SCR module and the DPF into a single module. U.S. patent application publication no. 2014/0234118 allegedly teaches an exhaust treatment system having a DOC module, a DPF module downstream of the DOC module, and a combined SCR and DPF module that is downstream of the DPF module. The DPF module filters particles falling within a first size range, and the combined SCR and DPF module filters particles falling within a second size range, where the particles in the first size range are generally smaller than the particles in the second size range.